Bulk and composite structural ceramics can be fabricated by forming particulate matter into a desired shape (pellet, plate, cylinder, ring, sphere, tube, etc.) and then by subjecting the formed body to high temperatures so as to induce densification. The process of creating objects from powders, including ceramic powders, is based on atomic diffusion and referred to as sintering. In most sintering processes, the powdered material is held in a mold and then heated to a temperature below the melting point, typically around ⅔ of the melting temperature of the ceramic of interest. The atoms in the powder particles diffuse across the boundaries of the particles and via mass transport of the particles create a dense solid piece. Ceramics cannot be cast as one does with metals because of their very high melting temperatures. However, because the sintering temperature does not have to reach the melting point of the material, sintering is often chosen as the shaping process for materials with extremely high melting points.
Although, sintering is effective in reducing the porosity and enhancing properties such as strength, electrical conductivity, translucency and thermal conductivity, of the ceramics, it is not perfect. Sintered ceramics still maintain some porosity which may be detrimental to their physical properties and limit their usability. The foremost requirement for any commercial sintering process is maximum density at the lowest temperature and shortest time possible.
Moreover, sintering of high melting temperature ceramics have traditionally been done using pressure assisted methods such as hot pressing or hot isostatic pressing (HIP) by employing pressures of up to ˜250 MPa. For example, the typical hot pressing cycle for the sintering of boron carbide (B4C) involves temperatures in the range of 1800-2200° C. for up to 40 hours under atmosphere control. Due to the need of specialized equipment and the extreme conditions required using pressure assisted methods, sintering of ultrahigh melting temperature ceramics is cost prohibitive and difficult. Furthermore, most carbides, nitrides and boride decompose into their constituent oxides and require additional preventive steps during sintering at high temperatures mainly by controlling the atmosphere. Thus, the higher the temperature the more difficult it is to sinter the nonoxide ceramics.
U.S. Patent Publication No. 2013/0085055 entitled “Methods of Flash Sintering,” (incorporated herein by reference in its entirety) describes a process of “flash densification” of dielectrics and semiconducting oxide ceramics, such as Al2O3, Y2O3-doped ZrO2, MnCoO4 and SrTiO3. By virtue of the electronic structure in such materials, a large enough electric field can be maintained across the specimen while heating the ceramic to high temperatures. The effect of the electric field has been attributed to various mechanisms, including avalanche of charged defects, which all require the build-up of very high electric fields inside of the material. However, in nonoxide ceramics a very high electric field cannot be maintained inside the material due to electric conductivity of such materials and as governed by the laws of electromagnetism.
In view of the foregoing, a solution which overcomes the above-described inadequacies and shortcomings in preparing low porosity nonoxide ceramics is desired. In particular, it would be desirable to develop a sintering method of ultrahigh temperature covalent non-oxide ceramics at low temperatures without applied pressure and at very short periods of time.